Thats a very weird problem you're having. You said it worked for 3 months and then started doing this?
I've been running mine since this guide was posted, but never had any such development.
Do you see the mails coming in through the mailwatch interface? Also, are there any errors in your mail.log while this problem is taking place? Post your mail.log, atleast the last section of it so that I can take a look. Also, post your main.cf and master.cf files.
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